The ringdown and shadow of the astrophysically significant Kerr Black Hole (BH) are both intimately connected to a special set of bound null orbits known as Light Rings (LRs). Similarly, ultracompact objects with LRs but without an horizon could mimic BHs in some of their strong gravity phenomenology. However, are such ultracompact objects dynamically viable? Stationary and axisymmetric ultracompact objects that can form from smooth, quasi-Minkowski initial data must have at least one stable LR, which has been argued to trigger a spacetime instability. Until recently its development and fate had been unknown. We confirm that the LRs indeed triggered an instability in fully non-linear numerical evolutions of ultracompact bosonic stars free of any other known instabilities. The results shows that the LR instability can be effective in destroying horizonless ultracompact objects that could be plausible BH imitators.
Paulo Brás, Paulo Silva, Jaime Silva